The principal purpose of this trial is to assess the potential for the essential trace element selenium (Se) to inhibit the progression of prostate cancer. The rationale for this trial is based on the results of the Nutritional Prevention of Cancer (NPC) Trial, our double blind randomized trial which observed a 63 percent reduction in prostate cancer incidence occurred during the initial 10 years of follow up in the patients receiving 200 mug of Se compared io the placebo group 1. The trial will randomize patients to either placebo or one of two Se dosages, 200 mug, and 800 mug/d. The primary endpoints is the velocity of the primary serum marker of prostate cancer progression, prostate- specific antigen (PSA). Additional endpoints are time to disease progression, initiation of hormone therapy and time to documented metastatic disease. Safety endpoints for the trial include onset of early mild symptoms of Se toxicity as well as significant changes in liver and kidney enzyme levels. The trial will randomize at least 220 patients, in order to have 80 percent power to detect a 50 percent decrease in the velocity of PSA with an alpha of 0.05, during an average of 45 months of follow up. The treatment effect of 50 percent was selected because it is half the treatment effect observed in the NPC trial after a 2 year treatment lag (RR=0.25).